Ive had my 2007 CR250 for about 4 years now... its been my test mule for all sorts of engine and suspension modifications (with varying degress of success!)... its had more rebuilds than i can remember and i have a shed full of spares for it so rather than laying money down on a new bike i thought id just modify the %$#@ out of this one. Its getting a full on big bore/stroker/long rod build with an ignitech ignition to control the Powervalve, powerjet (already has a pwk38 on it), and ignition curve.
The plan is:
68.5mm bore (up from 66.4)
76mm stroke (up from 72)
130.3mm rod (up from 125.3)
Flat top piston
Toriodal combustion chamber
To be honest as i type this im already planning the next stage of this build which will take it over 300cc but more on that later...
Im still waiting on the stroker crank which should be here in a couple of weeks
With the cylinder, I was about to bore and plate one of my spare cylinders and then found a cheap, freshly plated 2003 68.5mm Eric Gorr cylinder, head, powervalve and new piston for about the same cost as plating here, so the decision was easy... get the EG stuff... ill still need to modify the porting layout to work with the rod/stroke changes but it gives me a spare cylinder, head and powervalve... which gives me some spares to play with the 300+ idea...
Like lambs to the slaughter... EG cylinder and head on the left, others are 2007 cylinders and head
Engine is already out of the bike, partly cleaned, ready for mocking up and finalising porting
In order to make the cylinder work with the longer rod i needed to make a spacer plate to go between the cylinder and cases... I used a stock base gasket as a template and modelled it using Mastercam X4 CAM software... machined one up the other day out of 8mm aluminium plate... worked a treat on the new cnc mill...
The final thickness will be worked out later once i have the crank, but it will end up around 5mm thick with stock base gaskets either side
I still have to design the final toroidal combustion chamber but I already know the basics of what its going to be... I have done several tests using the CAD/CAM software and its great to design exactly what you need and check volumes before cutting anything... then send the g-code file to the mill and it just spits out a perfect copy in aluminium, way easier (once youve learned the software! which has taken a lot of work) and more accurate than doing it by hand on a lathe which is what ive done up until now. Going to do some experimenting with squish clearance and compression to see what i can get out of the toroidal head shape... a little dyno time will be in order once its running. The 02-04 head has a nice low plug height which will be perfect for machining into the final design on the cnc mill...
Because this first stage of the project is using a 2003 cylinder i need to machine up a flange to fit the 2007 male ended pipe to the 2003 cylinder, which normally has a female ended pipe. Will do this in the next couple of days using the 2003 exhaust flange gasket as a template for the basic shape
the new flange will bolt onto the 2003 cylidner and fit the 2007 pipe inside
Ill work out the final porting numbers once i get the crank but its been interesting to look at Eric Gorrs work...this particular cylinder really has had very little done to it other than a few knife edges
and a tiny amount of grinding at the top of each transfer port. The rest of the duration changes have just happened naturally as a result of boring the cylinder the extra 2.1mm. 
EG cylinder
Mould of the transfer ducts
Finally, the Ignitech ignition... theres no pint showing a photos of it cause its just a small balck box and im sure many of you are already familiar with them... but it has some magic inside!! Ive already had it hooked up to my bike before i took the engine out... its awesome to connect the bike up the the laptop and make changes to the engine as its running, just love it!! There are some gains to be had with a stock CR just by changing the powervalve opening map... in stock form the powervalve flutters heaps and using the ignitech it is smooth as silk, and you can better match valve opening to rpms.
Ill be using the ignitech to run the ignition curve, the exhaust power valve and the keihin powerjet. Plus ill be utilising the potentiometer control for the acceleration threshold, with a small dial mounted on the bars
This build will probably take another 3-4 weeks id think and to say i am excited is an understatement:D
I race offroad and my interest is in broad power, not just peak hp so ill be doing everything with an eye to keeping it as ridable in the sloppy wet stuff as possible... in saying that i think 57ish crank hp will be realistic but time will tell... the software tells me that a bit more is possible but i'm not sure it'll suit my riding... then again its always fun to aim high then i can always detune it with the ignition.
